The present invention relates to a multicore fiber that is preferable to the case of multiplexing and demultiplexing light beams in different modes.
In optical communications using optical fibers, multimode communications are known, in which information is superposed on a light beam in the LP01 mode (the fundamental mode) as well as information is superposed on LP mode light beams higher than the fundamental mode, such as the LP11 mode for information communications. In multimode communications, a plurality of LP mode light beams propagated through one core is demultiplexed to a plurality of optical fibers, or LP mode light beams different from each other, which are propagate through a plurality of optical fibers, are multiplexed on one optical fiber.
Non-Patent Literature 1 below describes that in the case in which the same LP mode light beams are propagated through a set of optical fibers, the modes of the light beams propagated through the optical fibers are converted into LP modes different from each other, and then the light beams are multiplexed with each other.
A mode multiplexer/demultiplexer described in Patent Literature 1 below has two waveguides. In these waveguides, a first waveguide propagates specific LP mode light beams, and a second waveguide propagates one of the specific LP mode light beams and an LP mode light beam different from that one of the specific LP mode light beam. In the case in which these waveguides propagate the same specific LP mode light beams, the propagation constant of that one of the specific LP mode light beams in the first waveguide is matched with the propagation constant of the LP mode light beam different from that one in the second waveguide. With this configuration, the specific LP mode light beam in the first waveguide is multiplexed as the different LP mode light beam in the second waveguide. The second waveguide originally propagates the specific LP mode light beam. Consequently, the second waveguide propagates the original specific LP mode light beam and the specific LP mode light beam having been propagated through the first waveguide. Thus, with no use of any mode converter, in the case in which the same LP mode light beams are propagated through a plurality of optical waveguides, these light beams can be propagated through one waveguide as in light beams in different modes.    [Non-Patent Literature 1] An Li et al., “Low-Loss Fused Mode Coupler for Few-Mode Transmission”, OFC, OTu3G4 (2013)    [Patent Literature 1] JP-A-2013-37017